The invention relates to a transponder system for monitoring the locations of objects over relatively large distances.
It is an object of the invention to provide such a system which can be made available at relatively low cost and which is arranged so that each transponder can, in response to the receipt of an interrogating signal, transmit to a monitoring station a response peculiar to that transponder, or a group of transponders, and which response can be easily altered to take account of changing circumstances.